1. Field of the Invention
The invention relates to a configuration for operating an optical transmission or reception module at high data rates of up to 10 Gbits/s.
It is known practice to use optical transmission or reception modules in so-called TO (transistor outline) packages at data rates of up to 622 Mbit/s. TO packages are standard packages, known in the prior art, for optical transmission or reception modules whose form is similar to the package of a (conventional) transistor but which have a glass window for the entry and exit of light on the top side. The name coaxial laser modules is given to modules which contain a TO laser module and in which a glass fiber is coupled to the TO laser module.
Such a TO package, for example known from WO 99/57594, is shown schematically in FIG. 3. The TO package 1 contains a transmission or reception module 2. The transmission module 2 is preferably arranged on a silicon substrate 3. The essential components of the transmission module 2, which is also called a laser submount, are a laser chip 21, deflection prisms 22, 23 on both sides of the laser chip 21, a coupling lens 24 and a monitor diode 25.
The TO package 1 has a baseplate 100 having four electrical bushings, only two of which are shown. The baseplate 100 has an upper side 101 and a bottom side 102. The RF signal (data signal) is routed into the interior of the package via an insulated, for example glazed, pin 42 and is electrically connected in the interior of the package to the laser chip 21 by means of bonding wires or strips. A further bushing pin 41 is used to supply the laser module with bias current or, if the bias current is supplied together with the RF signal via the pin 41, to connect it to ground. The pins (not shown) are used for controlling the monitor diode 25.
The window in the TO package 1 is used for coupling to an optical fiber which launches light from the laser chip 21 or, if the arrangement is of basically the same design in the form of a reception module, outputs light for detection by the reception chip.
The advantage of using TO packages is low package costs and established production facilities which permit large numbers to be produced at low production costs. However, when high data rates are used, the problem which arises is that TO packages have poor RF properties as a result of comparatively long bonding wires in the TO package, the bushing capacitance of the glazed package bushings and the generally undefined RF properties of the connection between the TO connection legs and a driving board.
On account of the poor RF properties when using TO packages, transmission or reception modules in so-called butterfly packages have been used to date for high data rates. However, such butterfly packages are much more expensive than TO packages.
The present invention is based on the object of providing an arrangement for operating an optical transmission or reception module which uses the economical TO design and can also be used at high data rates of up to 10 Gbit/s.
Accordingly, the invention provides for the arrangement to have a TO package having an optical transmission or reception module and also to have a circuit board for making electrical contact with the electrical connections of the TO package, the circuit board having RF lines and the electrical connections being directly connected to the RF lines in an arrangement parallel to the plane of the board. In this context, the RF lines are preferably in the form of planar lines having a defined characteristic impedance, in particular in the form of microstrip lines, to which the electrical connections are directly soldered in a parallel position.
Connecting the connection legs of the TO package directly to the RF lines in a parallel arrangement achieves good field matching between the RF line and the TO bushings, with the result that the RF properties of the arrangement are improved.
The use of RF lines having a defined characteristic impedance makes it possible to obtain an optimum match between the characteristic impedance of the RF lines and the impedance of the transmission or reception module. In this context, a match is produced, in particular, for the frequency-dependent impedance of the module""s transmission or reception element and of its submount in the TO package, for the bonding connections to the connection pins of the transmission or reception module and for the characteristic impedance of the bushings for the electrical connections or pins of the TO package.
The RF lines on the board are preferably in the form of microstrip lines or coplanar lines. Other planar RF lines may also be used, however.
If the transmission module has a directly modulated laser, the characteristic impedance of the RF lines is preferably 30-50 ohms. If the transmission module has an electrical absorption modulator (EAM) which RF-modulates an inherently constant laser signal in line with a radiofrequency data signal, the characteristic impedance of the RF lines is preferably 50-80 ohms.
In a further aspect of the invention, provision is made for the RF circuit board to have a matching circuit for matching the impedance between the transmission or reception module and a driver or amplifier circuit arranged on the board. This ensures particularly effective and fault-free matching of the usually low-impedance semiconductor laser (typically 3 to 5 ohms) to the impedance of a driver circuit (usually 25 or 50 ohms), as a result of which the radiofrequency properties of the laser module are significantly improved.
If the reception module has a photodiode as reception chip, this photodiode usually has an associated preamplifier. In this case, the matching circuit ensures matching between the preamplifier and the RF line or data line situated on the RF board.
In this context, for the case in which the transmission module has a semiconductor laser and this semiconductor laser has signals from a driver circuit arranged on the RF board applied to it, provision is preferably made for the matching circuit to be designed for differential (symmetrical) driving of the transmission module by the driver circuit. A reception module may likewise be designed with differential (symmetrical) outputs on the preamplifier.
Alternatively, the matching circuit is designed for single-ended driving of the transmission module. However, the advantage of symmetrical driving over single-ended driving is that a higher signal swing is available for modulating the semiconductor laser, on account of the use of both driver outputs.
The matching circuit preferably has at least one resistor which matches the semiconductor laser to the impedance of the driver circuit.
In one preferred development of the invention, a bias circuit for generating a bias for the transmission or reception module is integrated onto the RF circuit board. In this case, the bias circuit preferably has a broadband RF inductor. This is used for RF blocking of the bias current connection of the semiconductor laser. By way of example, the inductor is a coil having a ferrite core.
The bias circuit and the matching circuit are preferably integrated into one circuit. In one preferred embodiment, this circuit has:
at least one first resistor,
an RF inductor which is connected to a voltage source and applies a bias to the semiconductor laser via the first resistor, and
a further resistor, whose one connection is connected to ground and whose other connection is connected to the first resistor.
In this context, a second resistor, connected in series with the first resistor, is preferably also provided, one connection of the further resistor being connected between the junction point of the two series-connected resistors.
Preferably, the inductor is additionally connected to ground via a relatively small capacitance. This ensures better blocking of the RF signal from the bias supply. In this case, the capacitance is arranged on the xe2x80x9ccold sidexe2x80x9d of the inductor.
In one preferred aspect of the invention, if the lines of the matching circuit and/or bias circuit are in the form of planar RF lines and the components used in the circuit are produced using SMD (Surface Mounted Device) technology, that is to say can be fitted directly onto the printed circuit board, the planar RF paths are used as solder pads for the SMD components. This avoids the components being attached to the board by means of solder pads which are otherwise necessary. The solder pads or the area formed by them would change the characteristic impedance of the interconnects. In particular, the solder pads would form an excessive additional capacitance which would hinder realization of the desired bandwidth of 10 Gbit/s.
In this context, microstrip lines are preferably used as planar lines, and the microstrip lines are used as the solder surface for the SMD components.
In one preferred development of the invention, the RF circuit board is a two-layer board, one connection of the TO package being connected to RF lines on the top side of the board, and a second connection of the TO package being connected to a ground surface on the underside of the board. If the TO package has more than two electrical connections, for example when a monitor diode is additionally used, the circuit board is preferably a multilayer, in particular four-layer, board, with the connections of the pins for the monitor diode preferably being situated on the underside of the multilayer board and being decoupled from the RF lines by the ground plane.
The TO package is advantageously arranged directly at the edge of the board. Accordingly, contact is immediately made between the electrical connections of the TO package and the RF lines on the circuit board, and hence no significant reflection points arise for the RF signal.
To improve the RF properties of the TO package, provision is preferably made for the bonding wires in the TO package to be kept as short as possible in order to keep down the total inductance. To produce small inductances, parallel bonding connections or multiple bonds may be provided.
For the case in which the optical transmission or reception module is provided with a receiver chip, a further preferred measure to improve the RF properties of the TO module involves matching the impedance of the package bushing of the TO package to the value of the impedance at the output of the receiver chip.
Furthermore, provision is preferably made for a short piece of a stripline having a high characteristic impedance to be added at the output of the TO package. The effect achieved by this is that the frequency response of the TO package is raised.